The overall objective of the proposed project is to further our understanding of the connectivity and maturation of the inhibitory circuit in the hippocampus. Until recently, it was believed that the activation of inhibition in the hippocampus was dependent upon glutamate-mediated excitation, via feedforward and feedback pathways. Our recent studies have demonstrated that inhibitory interneurons can also be recruited through two pathways which do not require glutamate- mediated neurotransmission: (1) via the depolarizing action of GABA on BABA/A receptors from synaptically connected interneurons and (2) via presumed electrotonic coupling. The proposed study will further characterize these novel, glutamate-independent inhibitory synchronization processes. In vitro experiments on the hippocampal slice will be performed to address three major issues. These are (1) confirmation of electrotonic coupling among a subpopulation of interneurons, (2) analysis of interneuron morphology to determine structural correlates for the functional differentiation of interneurons into various subgroups and (3) determination of the developmental profile of these glutamate-independent synchronization processes in the immature brain. All experiments will involve intracellular recording from identified inhibitory neurons and, in most cases, CA3 pyramidal cells. Electrotonic coupling will be verified directly, using paired recordings, and supportive data will be gathered using intracellular dye markers to uncover dye-coupled cells, application of volatile anesthetics and modification of the extracellular milieu (both procedures designed to disrupt gap junctions). Morphological analysis of interneurons will be performed using the intracellular marker Neurobiotin, with simultaneous electrophysiological analyses of intrinsic properties, synaptic and electrotonic coupling. The developmental studies will assess both the intrinsic maturation of interneurons and the maturation of the inhibitory circuit as a whole, as related to its output onto pyramidal cells. The inhibitory circuit plays an integral role in the normal function of the hippocampus, and is essential for limiting the propagation of abnormal activity in conditions such as epilepsy. Our recent studies demonstrate that the inhibitory circuit is much more complex than previously realized. The experiments in the proposed study will examine this complexity so that we can further our understanding of the normal and abnormal network function in the hippocampus.